The invention can find a most beneficial application in devices intended for making horizontal, inclined and vertical holes in the ground being compacted during trenchless laying of underground communication lines under highways, earth embankments and other structures.
The machine according to the invention can also be used for driving piles, down-the-hole soil compacting, as well as for forcing tubes and other structural elements in the ground.
A widespread use has found self-propelled pneumatic percussive action machines for driving holes in the ground by soil compacting. The principal working member of such a machine is a cylindrical housing with the interior thereof accommodating a hammer and an air-distributor. Used as a working medium is compressed air supplied along a flexible hose from a mobile compressor unit. During operation the compressed air fed to the working chambers through the air distributor makes the hammer reciprocate axially to deliver an impact on the housing. Under the action of the impacts the housing is forced into the ground in a pile-like manner, whereby a substantially straight well or hole is formed with smooth, soil-compacted walls. The diameter of such a hole equals the diameter of the percussive action machine.
Because retrieval of percussive action machines from the soil due to failure is very often difficult if not impossible (when making holes under railways or airport runways), they must be sufficiently reliable and have a long service life. Another important characteristic is their efficiency determined mostly by the impact power they produce.
There is known a machine for making holes in the ground (cf., USSR Inventor's Certificate No. 227,198. IPC E 02 F 5/18) which comprises a housing accommodating a reciprocable hammer defining inside the housing a forward stroke chamber continuously communicating with an air-feeding line, and a return stroke chamber alternately communicating with the forward stroke chamber and with the outside through a conduit means provided in the hammer in the form of a bore made in the hammer to extend perpendicularly to the longitudinal axis thereof, and an air-distributor in the form of a sleeve having holes.
The forward stroke chamber is defined here by the hammer and air-distributor and disposed in an axial bore made inside the hammer, which makes it necessary to use a hammer with rather thin walls.
Inherent in the aforedescribed percussive action machine is a disadvantage of relatively short service life, because of insufficient hammer wall thickness and stress concentrations in the hammer where its configuration suddenly changes at the point of termination of the axial bore, as well as because of the provision of the radial holes in the walls of the hammer (wherethrough the return stroke chamber defined by the outer surface of the hammer and the inner surface of the housing communicates with the forward stroke chamber).
For a greater length of the forward stroke of the hammer the return stroke chamber is not communicated with the outside, whereby a counterpressure is developed therein to exert a decelerating action on the hammer and reduce its impact power.
In turn, impact power of the machine cannot be increased by extending the length of the working stroke of the hammer. Therefore, a major structural parameter governing the impact power developed by the machine is the diameter of the hammer and, consequently, the diameter of the machine. However, such an increase in the diameter results in an increased bulk of the machine and resistance to its travel through the soil to affect the speed of hole making.
There is also known a machine for making holes in the ground (cf., "Gornye machiny"--Mining Machines, in Russian, Collection of Reports, AN SSSR, Sibirskoe otdelenie, Institut gornogo dela, Novosibirsk, 1980, pp. 14 to 20) comprising a cylindrical housing accommodating a reciprocable hammer which defines inside the housing a forward stroke chamber continuously communicating with an air-feeding line, and a return stroke chamber communicating alternately with the forward stroke chamber and with the outside through a conduit means provided in the hammer and having the form of a hole perpendicular to the longitudinal axis of the hammer, the hammer also having an air-distributor fashioned as a sleeve with holes and a tubular control valve secured in the housing and movable relative to the sleeve.
The provision of the movable tubular control valve makes it possible to increase the length of work stroke of the hammer (to consequently obtain a higher impact power and improved machine efficiency at the same outer diameter) because the hammer is returned through not only the expansion of compressed air in the return stroke chamber, but also due to the compressed air tending to occupy this chamber for a part of the return stroke of the hammer equal to the length of travel of the tubular control valve.
However, because of similar stress concentrations as is the case with the machine described in USSR Inventor's Certificate No. 227,198, and thin walls of the hammer, this prior art machine has a relatively short service life.
For a portion of the forward stroke of the hammer equal to the length of the tubular control valve a counter pressure is developed in the return stroke chamber exerting a braking effect on the hammer and reducing the impact power.
One condition for stable operation of the machine is simultaneous movement of the tubular control valve and hammer during the travel of the control valve from one extreme position to the other.
On the other hand, since such simultaneous movement is ensured by the forces of friction in action between the hammer and tubular control valve varying widely under impact and vibration loads, this condition can be met in practice with difficulty.
There is further known a percussive action machine for making holes in the ground (cf., USSR Inventor's Certificate No. 531,907; IPC E 02 F 5/18) comprising a cylindrical housing accommodating a reciprocable hammer defining inside the housing a forward stroke chamber continuously communicating with an air-feeding line, and a return stroke chamber communicating alternately with the forward stroke chamber and with the outside through a conduit means in the hammer provided with an air-distributor in the form of a sleeve with holes secured in the housing, and a tubular control valve movably arranged relative to the sleeve for opening and closing the holes in the sleeve, as well as a locking means for moving the control valve and setting it in two extreme positions, in one of which the return stroke chamber communicates through the conduit means in the hammer and through the air-distributor with the outside, whereas in the other position the return stroke chamber communicates with the forward stroke chamber.
The sleeve of the air-distributor comprises inlet and discharge passages defined by two coaxially-arranged tubes, the sleeve being disposed inside an axial bore of the hammer. During the forward stroke of the hammer compressed air is admitted both to the return stroke chamber and to the chamber formed by the end faces of the hammer and air-distribution sleeve, whereby the pressure of compressed air acts on the entire cross-sectional area of the hammer.
Compressed air is admitted to the chamber formed by the end faces of the hammer and sleeve and discharged thereform by opening and closing ports of the sleeve by the tubular control valve.
The forward stroke chamber is provided in the axial bore of the hammer, whereas the conduit means has the form of holes in the wall of the hammer, these holes causing substantial stress concentrations. In addition, the eleborate configuration of the hammer and the large diameter of the axial bore (due to the accommodation of the sleeve having inlet and discharge passages) affect the service life of the hammer.
Because through the greater length of the forward stroke of the hammer the return stroke chamber is not in communication with the outside, a counterpressure tends to develop therein to exert a decelerating effect on the hammer. In addition, the return travel of the hammer is effected exclusively thanks to the expansion of compressed air in the return stroke chamber. In other words, the prior art machine fails to make use of the energy resulting from the occupation of the return stroke chamber by compressed air. All this reduces the impact power of the machine and its efficiency.
It is an object of the present invention to improve the efficiency and reliability of a percussive action machine for making holes in the ground.
Another object is to simplify the machine structurally and increase the impact power developed by its hammer.